Interpretive Summary: The growth of varroa mite populations was highly variable among years during a 10-year period (1993¿2002) in Baton Rouge, LA. Growth of mite populations was monitored in colonies of bees with queens from many sources that had not been selected for resistance to varroa. In some years, population growth was much slower than had been found in typical years. These reduced growth rates correlated to periods of hot and dry weather. Years with the lowest growth rates occurred during a period (1998-2000) of significant drought in Louisiana when rainfall was lower than normal and air temperatures were warmer than normal. Research by others has shown that high temperatures and extremes in relative humidity within the broodnest of a colony of bees will reduce the number of varroa mites that lay eggs. We suggest that the lower growth of mite populations during the hot and dry periods was probably caused by a reduced reproductive rate for the mites. The impact of this study is that it suggests the need for sampling colonies of bees before treating with miticides. There were 3-4 years during the decade in which the growth rate for varroa mites was slow enough that damaging population levels were probably never reached during the spring-summer months (assuming that mite populations in the spring were low after treatment in the previous autumn). Excessive cost and exposure of colonies to insecticides could be avoided in those years if beekeepers monitored the actual pest population and only treated when necessary.

Technical Abstract:
The intrinsic growth rates for varroa mites (Varroa destructor) significantly varied among years during 1993--2002 in Baton Rouge, LA. Mite population growth was monitored in colonies of honey bees with queens obtained from different sources in the U.S. Queens were from sources that had not selectively been bred for varroa resistance. Mite populations were measured at the beginning and end of 10¿13 wk field tests that started in the late spring of each year. Multiple regression analyses showed that only the first two of the following regressors were linear predictors of r7, the intrinsic growth rate (wk-1). Regressors included (1) percentage of reproducing female mites, (2) proportion of total mites in capped brood, (3) mortality of mites in brood cells, (4) growth of the bee population, (5) capped brood area at the end of a test, and (6) duration of the test. Analysis of commonality indicated that the percentage of reproducing female mites explained about 26 % of the total variation in r7, and the proportion of total mites in capped brood explained 6 %. The joint expression of both variables accounted for another 4 %. Thus, residual error reflected most of the total variation in r7, which suggested possible climatic or environmental effects on mite growth. The lowest growth rates occurred in three consecutive years of substantial drought in Louisiana. Measures of ambient temperature and relative humidity correlated to growth of mite populations among different years. Reduced growth rates were probably the result of diminished reproductive rates by varroa mites during periods of hot and dry weather.